"It has been assumed that when you have a pod of 100 or 150 pilot whales strand that they were part of a single matrilineal social group and our results suggest that is not the case," says Baker.

Baker says mass strandings, which can involve more than 150 whales, are assumed to be natural events, unlike the stranding of small numbers of whales, which have been linked to human activity, such as naval sonar.

"The mass stranding of pilot whales and probably sperm whales and some of the other species has been documented for hundreds of years all the way back to Aristotle," says Baker.

"In general, without human assistance, these large mass strandings are almost always fatal."

The curious thing is whales appear to mass strand intentionally, and even after humans re-float them, they will often return to the beach.

Care-giving hypothesis

The most common species to mass strand are pilot whales and, according to one popular hypothesis, these events are based on kinship.

It is thought pilot whales live in pods consisting of individuals who are all descendents of a single maternal ancestor.

If just one whale becomes stranded (due to sickness or disorientation) then it is thought this triggers others in the extended family to follow so as to look after it.

If this "care-giving" hypothesis is right then all the whales in a mass stranding would be related to each other, says Baker.

But his latest research questions this hypothesis.

Genetic study

Baker and colleagues looked at the relatedness of 490 long-finned pilot whales (Globicephala melas) involved in 12 strandings around Tasmania and New Zealand.

Using analysis of mitochondrial DNA, which is inherited through the maternal line, the researchers revealed the stranding groups were descended from more than one ancestral mother.

They also found the relatedness of individuals didn't determine how close they were to each other on the beach.

"You'd expect to have a cluster of individuals that were of close relatives supporting a central individual that might be the matriarch. We just didn't see that," says Baker.

"Mothers and calves were very often widely separated on the beach," he adds. "And in some cases we had dependent calves dead on the beach, and no mother that matched those calves dead on the beach."

Baker says the latest findings do not question a matrilineal social organisation amongst pilot whales, but it suggestst this does not seem to extend across the entire group in a mass stranding.

He says the findings suggest that unrelated groups of pilot whales are coming together at some stage, possibly to mate or feed.

This could lead to competitive interactions and social disturbance that could in turn lead to the mass strandings, says Baker.

Other research suggests environmental factors such as changing water temperatures, natural magnetic fields or shallow waters that confuse the whales' sonar also play a role in strandings.

Implications for rescue

The latest findings have implications for rescue efforts, says Baker.

He says rescuers have tended to assume that a mature female next to a dependent calf is its mother and refloat the two together, but sometimes this fails and one or both return to the beach.

"People have speculated that animals restrand because they can still hear their kin vocalising on the beach," says Baker.

He says until more research is done on the behaviour of pilot whales before and after stranding these events are likely to remain a mystery.

"It is enigma. This is the word we've decided best describes it," says Baker.